Electrochemical reaction device

ABSTRACT

An electrochemical reaction device in an embodiment includes: a reaction unit including a first accommodation part to accommodate carbon dioxide and a second accommodation part to accommodate an electrolytic solution containing water; a reduction electrode to reduce the carbon dioxide; an oxidation electrode to oxidize the water; a power supply to pass current between the reduction electrode and the oxidation electrode; a pressure regulator to regulate a pressure in the first accommodation part; a reaction product detector to detect at least one of an amount and a kind of a substance produced at the reduction electrode; and a controller to control the pressure regulator based on a detection signal of the reaction product detector.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2018-175169, filed on Sep. 19, 2018; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an electrochemicalreaction device.

BACKGROUND

In recent years, there is a concern over depletion of fossil fuelresources such as petroleum and coal, and expectations of sustainablerenewable energy increases. From the viewpoint of such energy problems,environmental problems and so on, an artificial photosynthesistechnology is under development which electrochemically reduces carbondioxide using renewable energy of sunlight or the like to generate astockable chemical energy source. An electrochemical reaction devicerealizing the artificial photosynthesis technology includes, forexample, an oxidation electrode that oxidizes water (H₂O) to produceoxygen (O₂), and a reduction electrode that reduces carbon dioxide (CO₂)to produce a carbon compound. The oxidation reaction electrode and thereduction electrode of the electrochemical reaction device are generallyconnected to a power supply derived from renewable energy such as solarpower generation, hydroelectric power generation, wind power generation,geothermal power generation or the like.

The reduction electrode of the electrochemical reaction device isarranged, for example, to be immersed in water in which CO₂ is dissolvedor to be in contact with water which flows through a flow path and inwhich CO₂ is dissolved. The reduction electrode obtains reductionpotential for CO₂ from the power supply derived from renewable energyand thereby reduces CO₂ to produce carbon compounds such as carbonmonoxide (CO), formic acid (HCOOH), methanol (CH₃OH), methane (CH₄),ethanol (C₂H₅OH), ethane (C₂H₆), ethylene (C₂H₄), ethylene glycol(C₂H₆O₂) and the like.

A problem in the case of electrochemically reducing CO₂ using theabove-described renewable energy is that power is likely to fluctuatedue to the change in weather, wind condition or the like and the appliedvoltage to the reduction electrode is likely to fluctuate accompanyingthe change. The change in the applied voltage to the reduction electrodecauses successive fluctuations in production amount and composition of areduction product of CO₂ to be obtained. This becomes a factor todecrease the availability and utility value of the reduction product ofCO₂. As a method to solve the problem, a potentiostat capable ofapplying a constant potential to the reduction electrode to operate(three-electrode system) the electrochemical reaction device isexperimentally performed, but this loses the advantages owing to theutilization of the renewable energy and results in a problem in terms ofcost and efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an electrochemical reaction device in afirst embodiment.

FIG. 2 is a diagram illustrating a modified example of theelectrochemical reaction device in the first embodiment.

FIG. 3 is a diagram illustrating an electrochemical reaction device in asecond embodiment.

FIG. 4 is a chart illustrating a relation between a cathode potentialand a CO selection ratio in an electrochemical reaction device inExample 1.

FIG. 5 is a chart illustrating an example of temporal change in pressurein a cathode chamber in the electrochemical reaction device in Example1.

FIG. 6 is a chart illustrating an example of temporal change in cathodepotential in the electrochemical reaction device in Example 1.

DETAILED DESCRIPTION

An electrochemical reaction device in an embodiment includes: a reactionunit including a first accommodation part configured to accommodatecarbon dioxide, a second accommodation part configured to accommodate anelectrolytic solution containing water, and a diaphragm provided betweenthe first accommodation part and the second accommodation part; areduction electrode configured to reduce the carbon dioxide; anoxidation electrode configured to oxidize the water; a pressureregulator configured to regulate a pressure in the first accommodationpart; a reaction product detector configured to detect at least one ofan amount and a kind of a substance produced at the reduction electrode;and a controller configured to control the pressure regulator based on adetection signal of the reaction product detector.

Electrochemical reaction devices in embodiments will be describedhereinafter with reference to the drawings. Substantially the samecomponents are denoted by the same reference signs and descriptionthereof may be omitted in some cases in the embodiments described below.The drawings are schematic, and the relation between thicknesses andplane dimensions, ratios between the thicknesses of the parts and thelike may differ from actual ones.

First Embodiment

FIG. 1 is a view illustrating an electrochemical reaction device 1 in afirst embodiment. An electrochemical reaction device 1A illustrated inFIG. 1 includes: an electrochemical reaction cell 10 including areaction vessel 7 including a first accommodation part 3 configured toaccommodate a first electrolytic solution 2 containing CO₂, a secondaccommodation part 5 configured to accommodate a second electrolyticsolution 4 containing water, and a diaphragm 6, a reduction electrode(cathode) 8 arranged in the first accommodation part 3, and an oxidationelectrode (anode) 9 arranged in the second accommodation part 5; a powersupply 11 connected to the reduction electrode 8 and the oxidationelectrode 9; a first product separator 12 configured to separate areduction reaction product produced in the first accommodation part 3from the first electrolytic solution 2; a second product separator 13configured to separate an oxidation reaction product produced in thesecond accommodation part 5 from the second electrolytic solution 4; apressure regulator 14 configured to regulate a pressure in the firstaccommodation part 3; a reaction product detector 15 configured todetect at least one of an amount and a kind of a substance produced atthe reduction electrode 8; and a controller 16 configured to control thepressure regulator 14 based on a detection signal of the reactionproduct detector 15. Hereinafter, the units and so on will be describedin detail.

The reaction vessel 7 is separated into two chambers by the diaphragm 6capable of moving ions such as hydrogen ions (H⁺) and hydroxide ions(OH⁻) and the like, and has the first accommodation part 3 and thesecond accommodation part 5. The reaction vessel 7 may be made of, forexample, quartz white plate glass, polystyrol, polymethacrylate or thelike. A material transmitting light may be used for a part of thereaction vessel 7, and a resin material may be used for the remainder.Examples of the resin material include polyetheretherketone (PEEK),polyamide (PA), polyvinylidene fluoride (PVDF), polyacetal (POM)(copolymer), polyphenyleneether (PPE), acrylonitrile-butadiene-styrenecopolymer (ABS), polypropylene (PP), polyethylene (PE) and so on.

In the first accommodation part 3, the reduction electrode 8 is arrangedand the first electrolytic solution 2 is accommodated. The firstelectrolytic solution 2 functions as a reduction electrode solution(cathode solution) and contains carbon dioxide (CO₂) as a substance tobe reduced. The first electrolytic solution 2 may contain hydrogen ionsand is preferably an aqueous solution. In the second accommodation part5, the oxidation electrode 9 is arranged and the second electrolyticsolution 4 is accommodated. The second electrolytic solution 4 functionsas an oxidation electrode solution (anode solution) and contains water(H₂O) as a substance to be oxidized. The second electrolytic solution 4may be an alcohol aqueous solution, an aqueous solution of an organicsubstance such as amine or the like.

It is possible to change the amount of water and electrolytic solutioncomponents contained in the first and second electrolytic solutions 2, 4to change the reactivity so as to change the selectivity of thesubstance to be reduced and the proportion of the chemical substance.The first and second electrolytic solutions 2, 4 may contain redoxcouples as needed. Examples of the redox couple include Fe³⁺/Fe²⁺ andIO³⁻/I⁻.

The first and second accommodation parts 3, 5 may include space partsfor accommodating gas contained in the reactant and the product. To thefirst accommodation part 3, a first liquid supply flow path 17 forsupplying the first electrolytic solution 2 is connected, and the firstproduct separator 12 is connected through a first gas and liquiddischarge flow path 19. To the second accommodation part 5, a secondliquid supply flow path 18 for supplying the second electrolyticsolution 4 is connected, and the second product separator 13 isconnected through a second gas and liquid discharge flow path 20.

The first electrolytic solution 2 and the second electrolytic solution 4may be electrolytic solutions containing different substances or may beelectrolytic solutions containing the same substance. In the case wherethe first electrolytic solution 2 and the second electrolytic solution 4contain the same substance and the same solvent, the first electrolyticsolution 2 and the second electrolytic solution 4 may be regarded as oneelectrolytic solution. Besides, the pH of the second electrolyticsolution 4 is preferably higher than the pH of the first electrolyticsolution 2. This makes the hydrogen ions, the hydroxide ions and so oneasy to move. Further, the liquid junction potential due to thedifference in pH can effectively promote the oxidation-reductionreaction.

The first electrolytic solution 2 is preferably a solution with highabsorptance of CO₂. The existing form of CO₂ in the first electrolyticsolution 2 is not always limited to a state of being dissolved therein,but CO₂ in an air bubble state may exist to be mixed in the firstelectrolytic solution 2. Examples of the electrolytic solutioncontaining carbon dioxide include aqueous solutions containinghydrogencarbonates and carbonates such as lithium hydrogen carbonate(LiHCO₃), sodium hydrogen carbonate (NaHCO₃), potassium hydrogencarbonate (KHCO₃), sodium carbonate (Na₂CO₃), potassium carbonate(K₂CO₃), and cesium hydrogen carbonate(CsHCO₃), phosphoric acid, boricacid, and so on. The electrolytic solution containing carbon dioxide maycontain alcohols such as methanol, ethanol, acetone and the like, or maybe an alcohol solution. The first electrolytic solution 2 may be anelectrolytic solution containing a CO₂ absorbent that lowers thereduction potential for CO₂, has high ion conductivity, and absorbs CO₂.

As the second electrolytic solution 4, a solution containing water(H₂O), for example, an aqueous solution containing an arbitraryelectrolyte may be used. The solution is preferably an aqueous solutionthat promotes the oxidation reaction of water. Examples of the aqueoussolution containing the electrolyte include aqueous solutions containingphosphate ion (PO4₂ ⁻), borate ion (BO₃ ³⁻), sodium ion (Na⁺), potassiumion (K⁺), calcium ion (Ca₂ ⁺), lithium ion (Li⁺), cesium ion (Cs⁺),magnesium ion (Mg²⁺), chloride ion (Cl⁻), hydrogen carbonate ion (HCO₃⁻), carbonate ion (CO₃ ⁻), hydroxide ion (OH⁻) and the like.

As the above-described electrolytic solutions 2, 4, for example, ionicliquids made of salts of cations such as imidazolium ions or pyridiniumions and anions such as BF₄ ⁻ or PF₆ ⁻ and in a liquid state in a widetemperature range, or aqueous solutions thereof may be used. Examples ofother electrolytic solutions include amine solutions such asethanolamine, imidazole, and pyridine, and aqueous solutions thereof.Examples of amine include primary amine, secondary amine, and tertiaryamine. The electrolytic solutions may be high in ion conductivity andhave properties of absorbing carbon dioxide and characteristics oflowering the reduction energy.

Examples of the primary amine include methylamine, ethylamine,propylamine, butylamine, pentylamine, hexylamine and the like.Hydrocarbons of the amine may be substituted by alcohol, halogen and thelike. Examples of amine whose hydrocarbons are substituted includemethanolamine, ethanolamine, chloromethylamine and the like.

Further, an unsaturated bond may exist. These hydrocarbons are also thesame in the secondary amine and the tertiary amine.

Examples of the secondary amine include dimethylamine, diethylamine,dipropylamine, dibutylamine, dipentylamine, dihexylamine,dimethanolamine, diethanolamine, dipropanolamine and the like. Thesubstituted hydrocarbons may be different. This also applies to thetertiary amine. Examples with different hydrocarbons includemethylethylamine, methylpropylamine and the like.

Examples of the tertiary amine include trimethylamine, triethylamine,tripropylamine, tributylamine, trihexylamine, trimethanolamine,triethanolamine, tripropanolamine, tributanolamine, tripropanolamine,triexanolamine, methyldiethylamine, methyldipropylamine and the like.

Examples of the cation of the ionic liquid include1-ethyl-3-methylimidazolium ion, 1-methyl-3-propylimidazolium ion,1-butyl-3-methylimidazole ion, 1-methyl-3-pentylimidazolium ion,1-hexyl-3-methylimidazolium ion and the like.

A second place of the imidazolium ion may be substituted. Examples ofthe cation of the imidazolium ion whose second place is substitutedinclude 1-ethyl-2,3-dimethylimidazolium ion,1,2-dimethyl-3-propylimidazolium ion, 1-butyl-2,3-dimethylimidazoliumion, 1,2-dimethyl-3-pentylimidazolium ion,1-hexyl-2,3-dimethylimidazolium ion and the like.

Examples of the pyridinium ion include methylpyridinium,ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium,hexylpyridinium and the like. In both of the imidazolium ion and thepyridinium ion, an alkyl group may be substituted, or an unsaturatedbond may exist.

Examples of the anion include fluoride ion (F⁻), chloride ion (Cl⁻),bromide ion (Br⁻), iodide ion (I⁻), BF₄ ⁻, PF₆ ⁻, CF₃COO⁻, CF₃SO³⁻, NO₃⁻, SCN⁻, (CF₃SO₂)₃C⁻, bis(trifluoromethoxysulfonyl)imide,bis(perfluoroethylsulfonyl)pimide and the like. Dipolar ions in whichthe cations and the anions of the ionic liquid are coupled byhydrocarbons may be used. Note that a buffer solution such as apotassium phosphate solution may be supplied to the accommodation parts3, 5.

For the diaphragm 6, a membrane capable of selectively allowing theanion or the cation to pass therethrough is used. This makes it possibleto make the electrolytic solutions 2, 4 in contact with the reductionelectrode 8 and the oxidation electrode 9 respectively electrolyticsolutions containing different substances, and to promote the reductionreaction and the oxidation reaction depending on the difference in ionicstrength, the difference in pH or the like. The diaphragm 6 can be usedto separate the first electrolytic solution 2 from the secondelectrolytic solution 4. The diaphragm 6 may have a function of allowingpart of ions contained in the electrolytic solutions 2, 4 in which boththe electrodes 8, 9 are immersed, namely, a function of blocking one ormore kinds of ions contained in the electrolytic solutions 2, 4. Thiscan differ, for example, the pH between the two electrolytic solutions2, 4.

As the diaphragm 6, an ion exchange membrane such as NEOSEPTA(registered trademark) of ASTOM Corporation, Selemion (registeredtrademark), Aciplex (registered trademark) of ASAHI GLASS CO., LTD.,Fumasep (registered trademark), fumapem (registered trademark) ofFumatech GmbH, Nafion (registered trademark) being fluorocarbon resinmade by sulfonating and polymerizing tetrafluoroethylene of E.I. du Pontde Nemours and Company, lewabrane (registered trademark) of LANXESS AG,IONSEP (registered trademark) of IONTECH Inc., Mustang (registeredtrademark) of PALL Corporation, ralex (registered trademark) of megaCorporation, Gore-Tex (registered trademark) of Gore-Tex Co., Ltd. orthe like can be used. Besides, the ion exchange membrane may be composedusing a membrane having hydrocarbon as a basic skeleton or a membranehaving an amine group in anion exchange. When the first electrolyticsolution 2 and the second electrolytic solution 4 are different in pH,the electrolytic solutions can be used while stably keeping their pHs byusing a bipolar membrane made by stacking a cation exchange membrane andan anion exchange membrane.

Other than the ion exchange membrane, for example, porous membranes of asilicone resin, fluorine-based resins (perfluoroalkoxyalkane (PFA),perfluoroethylene propene copolymer (FEP), polytetrafluoroethylene(PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidenefluoride (PVDF), polychlorotrifluoroethylene (PCTFE),ethylene-chlorotrifluoroethylene copolymer (ECTFE) and the like), andceramics, packing filled with glass filter, agar and the like,insulating porous bodies of zeolite and oxide and the like may be usedas the diaphragm 6. In particular, a hydrophilic porous membrane nevercauses clogging due to air bubbles and is thus preferable as thediaphragm 6.

The reduction electrode 8 is an electrode (cathode) that reduces carbondioxide (CO₂) to produce a carbon compound. The reduction electrode 8 isarranged in the first accommodation part 3 and immersed in the firstelectrolytic solution 2. The reduction electrode 8 contains, forexample, a reduction catalyst for producing the carbon compound by thereduction reaction of carbon dioxide. Examples of the reduction catalystinclude a material that lowers activation energy for reducing carbondioxide. In other words, a material that lowers an overvoltage when thecarbon compound is produced by the reduction reaction of carbon dioxidecan be exemplified.

For example, a metal material or a carbon material can be used as thereduction electrode 8. As the metal material, for example, a metal suchas gold, aluminum, copper, silver, platinum, palladium, zinc, mercury,indium, or nickel, or an alloy containing the metal can be used. As thecarbon material, for example, graphene, carbon nanotube (CNT),fullerene, ketjen black or the like can be used. Note that the reductioncatalyst is not limited to the above but, for example, a metal complexsuch as a Ru complex or a Re complex, or an organic molecule having animidazole skeleton or a pyridine skeleton may be used as the reductioncatalyst. The reduction catalyst may be a mixture of a plurality ofmaterials. The reduction electrode 8 may have, for example, a structurehaving the reduction catalyst in a thin film shape, a mesh shape, aparticle shape, a wire shape or the like provided on a conductivesubstrate.

Examples of the carbon compound produced by the reduction reaction atthe reduction electrode 8 include carbon monoxide (CO), formic acid(HCOOH), methane (CH₄), methanol (CH₃OH), ethane (C₂H₆), ethylene(C₂H₄), ethanol (C₂H₅OH), formaldehyde (HCHO), ethylene glycol (C₂H₆O₂)and so on though different depending on the kind or the like of thereduction catalyst. Further, at the reduction electrode 8, a sidereaction of producing hydrogen (H₂) by the reduction reaction of water(H₂O) may occur at the same time with the reduction reaction of carbondioxide (CO₂).

The oxidation electrode 9 is an electrode (anode) that oxidizes water(H₂) to produce oxygen. The oxidation electrode 9 is arranged in thesecond accommodation part 5 and immersed in the second electrolyticsolution 4. The oxidation electrode 9 contains an oxidation catalyst forH₂O as a substance to be oxidized. As the oxidation catalyst, a materialthat lowers activation energy for oxidizing H₂O, in other words, amaterial that lowers an overvoltage when oxygen and hydrogen ions areproduced by the oxidation reaction of H₂ is used.

Examples of the oxidation catalyst material include metals such asruthenium, iridium, platinum, cobalt, nickel, iron, manganese and thelike. Further, a binary metal oxide, a ternary metal oxide, a quaternarymetal oxide or the like can be used. Examples of the binary metal oxideinclude manganese oxide (Mn—O), iridium oxide (Ir—O), nickel oxide(Ni—O), cobalt oxide (Co—O), iron oxide (Fe—O), tin oxide (Sn—O), indiumoxide (In—O), ruthenium oxide (Ru—O) and the like. Examples of theternary metal oxide include Ni—Fe—O, Ni—Co—O, La—Co—O, Ni—La—O, Sr—Fe—Oand the like. Examples of the quaternary metal oxide include Pb—Ru—Ir—O,La—Sr—Co—O and the like. Note that the oxidation catalyst is not limitedto the above, but a metal hydroxide containing cobalt, nickel, iron,manganese or the like, or a metal complex such as a Ru complex or a Fecomplex may be used as the oxidation catalyst. Further, a plurality ofmaterials may be mixed together for use.

Further, the oxidation electrode 9 may be composed of a compositematerial containing both the oxidation catalyst and a conductivematerial. Examples of the conductive material include: carbon materialssuch as carbon black, activated carbon, fullerene, carbon nanotube,graphene, ketjen black, diamond and the like; transparent conductiveoxides such as indium tin oxide (ITO), zinc oxide (ZnO), fluorine-dopedtin oxide (FTO), aluminum-doped zinc oxide (AZO), antimony-doped tinoxide (ATO) and the like; metals such as Cu, Al, Ti, Ni, Ag, W, Co, Auand the like; and alloys each containing at least one of the metals. Theoxidation electrode 9 may have a structure having the oxidation catalystin a thin film shape, a mesh shape, a particle shape, a wire shape orthe like provided on a conductive substrate. As the conductivesubstrate, for example, a metal material containing titanium, titaniumalloy, or stainless steel is used.

The power supply 11 is to supply power to make the electrochemicalreaction cell 10 cause the oxidation-reduction reaction, and iselectrically connected to the reduction electrode 8 and the oxidationelectrode 9. The electric energy supplied from the power supply 11 isused to cause the reduction reaction by the reduction electrode 8 andthe oxidation reaction by the oxidation electrode 9. The power supply 11and the reduction electrode 8 are connected and the power supply 11 andthe oxidation electrode 9 are connected, for example, by wiring. Betweenthe electrochemical reaction cell 10 and the power supply 11, electricequipment such as an inverter, a converter, a battery and so on may beinstalled as needed. The drive system of the electrochemical reactioncell 10 may be a constant-voltage system or may be a constant-currentsystem.

The power supply 11 may be the commercial power supply, a battery or thelike, or may be a power supply that supplies electric energy obtained byconverting renewable energy. Examples of the power supply include apower supply that converts kinetic energy or potential energy such aswind power, water power, geothermal power, tidal power or the like toelectric energy, a power supply such as a solar cell including aphotoelectric conversion element that converts light energy to electricenergy, a power supply such as a fuel cell or a storage battery thatconverts chemical energy to electric energy, an apparatus that convertsvibrational energy such as sound to electric energy, and so on. Thephotoelectric conversion element has a function of performing chargeseparation by emitted light energy of sunlight or the like. Examples ofthe photoelectric conversion element include a pin-junction solar cell,a pn-junction solar cell, an amorphous silicon solar cell, amultijunction solar cell, a single crystal silicon solar cell, apolycrystalline silicon solar cell, a dye-sensitized solar cell, anorganic thin-film solar cell, and the like. The photoelectric conversionelement may be stacked on at least one of the reduction electrode 8 andthe oxidation electrode 9 inside the reaction vessel 7.

The reduction reaction product produced at the reduction electrode 8 issent through the first gas and liquid discharge flow path 19 to thefirst product separator 12. In the first product separator 12, carbonmonoxide (CO) being a gaseous product, or formic acid (HCOOH), methane(CH₄), methanol (CH₃OH), ethane (C₂H₆), ethylene (C₂H₄), ethanol(C₂H₅OH), formaldehyde (HCHO), ethylene glycol (C₂H₆O₂) or the likebeing a liquid product is separated from the first electrolytic solution2. When the reduction reaction of water (H₂O) being the side reaction ofthe reduction reaction of carbon dioxide (CO₂) occurs, hydrogen (H₂)produced therefrom is also separated from the first electrolyticsolution 2. Further, the oxidation reaction product produced at theoxidation electrode 9 is sent through the second gas and liquiddischarge flow path 20 to the second product separator 13. In the secondproduct separator 13, oxygen (O₂) being a gaseous product is mainlyseparated from the second electrolytic solution 4.

The first gas and liquid discharge flow path 19 is provided with thepressure regulator 14 that regulates the pressure in the firstaccommodation part 3. The pressure regulator 14 is arranged on the backpressure side of the first accommodation part 3. As the pressureregulator 14, for example, a variable throttle, a flow rate controlvalve or the like is used. More specifically, the flow rate of fluid(fluid including gas and liquid) flowing through the first gas andliquid discharge flow path 19 can be controlled to regulate the pressurein the first accommodation part 3. The pressure in the firstaccommodation part 3 is preferably set to a pressure which does notliquefy CO₂, and is concretely regulated in a range of 0.1 MPa or moreand 6.4 MPa or less. If the pressure in the first accommodation part 3is less than 0.1 MPa, the reduction reaction efficiency of CO₂ maydecrease. If the pressure in the first accommodation part 3 exceeds 6.4MPa, CO₂ is liquefied and the reduction reaction efficiency of CO₂ maydecrease.

Increasing or decreasing the pressure in the first accommodation part 3can control the amount and kind of the reduction reaction product to beproduced at the reduction electrode 8. In other words, regulating thepressure in the first accommodation part 3 to change the partialpressure of CO₂ being reacting species, thereby enabling regulation ofthe CO₂ amount near the reduction electrode 8. This can control thereduction potential of the reduction electrode 8. As will be describedbelow in detail, also in the case where the applied voltage to thereduction electrode 8 from the power supply 11 fluctuates, the reductionpotential of the reduction electrode 8 can be adjusted to apredetermined value. Accordingly, the fluctuation in the productionamount and the composition of the reduction reaction productaccompanying the fluctuation in the applied voltage to the reductionelectrode 8 is suppressed to enable stabilization of the productionamount and the composition of the reduction reaction product.

Further, as illustrated in FIG. 2, a first pressure regulator 141 isarranged on the back pressure side of the first accommodation part 3 anda second pressure regulator 142 may be additionally arranged on the backpressure side of the second accommodation part 5. The second pressureregulator 142 is provided at the second gas and liquid discharge flowpath 20. The provision of the second pressure regulator 142 enablesregulation of the differential pressure between the first accommodationpart 3 and the second accommodation part 5. This can suppress breakageof the like of the diaphragm 6 due to the differential pressure betweenthe first accommodation part 3 and the second accommodation part 5. Thedifference between the pressure in the first accommodation part 3 andthe pressure in the second accommodation part 5 (differential pressure)is preferably set to 0.5 MPa or less.

The regulation of the pressure in the first accommodation part 3 by thepressure regulator 14 is performed by sending a detection signal, whichrepresents at least one of the amount and the kind of the substanceproduced at the reduction electrode 8, detected by the reaction productdetector 15 to the controller 16 and controlling the operation of thepressure regulator 14 by the controller 16. The reaction productdetector 15 is not limited to an analyzer that performs gas analysis orliquid analysis on the substance produced in the first accommodationpart 3, but an electrode potential monitor that monitors the potentialof the reduction electrode 8, a voltage and current monitor thatmonitors at least one of the voltage and the current of the reductionelectrode 8 and the oxidation electrode 9, or the like can be used. Thereaction product detector 15 is electrically connected to the controller16, and the controller 16 is electrically connected to the pressureregulator 14. The reaction product detector 15 outputs a signal based onthe detection result to the controller 16.

The reaction product detector 15 illustrated in FIG. 1 includes anelectrode potential monitor that monitors the potential of the reductionelectrode 8 through a reference electrode 21. Since the electrodepotential of the reduction electrode 8 is one of factors that decide theamount of current and the Faradaic efficiency of the product, the amountand the composition of the substance produced from the reductionelectrode 8 can be recognized by monitoring the electrode potential. Inthe case of monitoring the electrode potential of the reductionelectrode 8, the reference electrode 21 is arranged in the firstaccommodation part 3 as illustrated in FIG. 1. The reaction productdetector 15 is connected to the reference electrode 21. The referenceelectrode 21 may be the one made of any material as long as it is madeof a material usable as an electrode material such as platinum, gold,silver, copper, SUS, carbon or the likes. It is also possible to use areference electrode 21 used for electrochemistry measurement, such as asilver-silver chloride electrode, a calomel electrode, a mercury-mercuryoxide electrode or the like.

An electrochemical reaction device 1A illustrated in FIG. 2 includes, asthe reaction product detector 15, an analyzer that performs at least oneof gas analysis and liquid analysis. The reaction product detector 15being the analyzer is arranged on the discharge side of the firstproduct separator 12 and directly analyzes the production amount and thecomposition of the reduction reaction product from gas and liquid beingthe separated substance. The analyzer as the reaction product detector15 is composed of an apparatus such as a gas chromatography, ahigh-performance liquid chromatography, or an ion chromatography capableof analyzing hydrocarbon in gas and liquid. The detection signal of theproduction amount and the composition of the reduction reaction productdetected by the analyzer is sent to the controller 16 to thereby controlthe operation of the pressure regulator 14. Note that in the case wherethe reaction product detector 15 detects the product by monitoring theelectrode potential of the reduction electrode 8 and the voltage andcurrent (cell voltage and current) of the electrochemical reaction cell,the composition and the amount of the product can be recognizedindirectly from the electrode potential or the cell voltage and currentby investigating in advance the relation (dependence) between thecomposition and the amount of the product, and, the electrode potentialor the cell voltage and current. As illustrated in a later-describedsecond embodiment, in the case of monitoring the cell voltage, thereaction product detector 15 is a signal outputting-type voltmeterconnected to the cell in parallel. In the case of monitoring the cellcurrent, the reaction product detector 15 is a signal outputting-typeammeter connected to the cell in series. The reaction product detector15 may have a form incorporated in the power supply 11.

The controller 16 is electrically connected to the pressure regulator 14and the reaction product detector 15. The controller 16 receives thedetection signal (data signal) from the reaction product detector 15 andoutputs a control signal to the pressure regulator 14. The controller 16stores in advance a request criterion of the data signal relating to thecomposition and the amount of the product, so that when the requestcriterion is not satisfied, the controller 16 outputs the control signalto the pressure regulator 14. The controller 16 is composed of acomputer such as a PC or a microcomputer, and arithmetically processesthe data signal from the reaction product detector 15 and controls theoperation of the pressure regulator 14 so that the potential of thereduction electrode 8 becomes a predetermined potential, therebyregulating the internal pressure of the first accommodation part 3.

Next, the operation of the electrochemical reaction device 1A will bedescribed. Here, a case of using water and an aqueous solutioncontaining carbon dioxide as the electrolytic solutions 2, 4 to reducecarbon dioxide to mainly produce carbon monoxide will be described. Whena voltage of an electrolysis voltage or higher is applied between thereduction electrode 8 and the oxidation electrode 9, the oxidationreaction of water (H₂O) occurs near the oxidation electrode 9 in contactwith the second electrolytic solution 4. As expressed in the followingExpression (1), the oxidation reaction of H₂ contained in the secondelectrolytic solution 4 occurs, and electrons are lost and oxygen (O₂)and hydrogen ions (H⁺) are produced. A part of the produced hydrogenions (H⁺) move through the diaphragm 6 into the first electrolyticsolution 2.

2H₂O→4H⁺+O₂+4_(e) ⁻  (1)

When the hydrogen ions (H⁺) produced on the oxidation electrode 9 sidereach the vicinity of the reduction electrode 8 and electrons (e) aresupplied to the reduction electrode 8 from the power supply 11, thereduction reaction of carbon dioxide (CO₂) occurs. As expressed in thefollowing Expression (2), CO₂ contained in the first electrolyticsolution 2 is reduced by the hydrogen ions (H⁺) moved to the vicinity ofthe reduction electrode 8 and the electrons (e⁻) supplied from the powersupply 11 to produce carbon monoxide (CO).

2CO₂+4H⁺+4e⁻→2CO+2H₂O   (2)

Note that the reduction reaction of CO₂ is not limited to the COproduction reaction but may be a production reaction of ethanol(C₂H₅OH), ethylene (C₂H₄), ethane (C₂H₆), methane (CH₄), methanol(CH₃OH), acetic acid (CH₃COOH), propanol (C₃H₇OH) or the like.

The reduction reaction by the reduction electrode 8 fluctuates dependingon the potential applied to the reduction electrode 8. For example,there is a case where H₂ gas is produced by the reduction reaction ofwater in addition to the above-described production reaction of CO gas.The rate between production amounts of CO gas and H₂ gas fluctuatesdepending on the potential applied to the reduction electrode 8, and theamount of H₂ gas mixed in CO gas being the main target product mayincrease in some cases. Besides, the kind or the like of the reductioncatalyst constituting the reduction electrode 8 is selected to make anorganic compound such as ethanol, ethylene, or ethane the main targetproduct, in place of CO gas in some cases. In such a case, theproduction amount of CO increases and the production amount of theorganic compound decreases depending on the potential applied to thereduction electrode 8. The fluctuations in composition and productionamount of the reduction reaction product accompanying theabove-described fluctuation in the applied voltage to the reductionelectrode 8 are monitored by the reaction product detector 15. Then, thepressure regulator 14 is controlled based on the monitoring result bythe reaction product detector 15, whereby the pressure in the firstaccommodation part 3 is regulated as described above. This adjusts thepotential of the reduction electrode 8 to be a desired potential, andadjusts the composition and the production amount of the reductionreaction product to desired states. Accordingly, it becomes possible tosuppress the successive fluctuations in production amount andcomposition of the reduction reaction product of CO₂ due to the changein the applied voltage to the reduction electrode 8 to thereby enhancethe availability and utility value of the reduction reaction product.

Second Embodiment

Next, an electrochemical reaction device 1 in a second embodiment willbe described referring to FIG. 3. An electrochemical reaction device 1Billustrated in FIG. 3 is different from the electrochemical reactiondevice 1A in the first embodiment in a contact type of gas containingCO₂ (simply described as CO₂ gas in some cases) or a first electrolyticsolution (cathode solution) containing CO₂ with the reduction electrode8, a contact type of a second electrolytic solution (anode solution)containing water with the oxidation electrode 9, and a connection typeof the reduction electrode 8 and the oxidation electrode 9 with thepower supply 11. The configurations of the units other than them, forexample, the concrete configurations of the reduction electrode, theoxidation electrode, the diaphragm, the first electrolytic solution, thesecond electrolytic solution, the power supply and so on, the separationof the product, the detection of the product, the pressure regulationbased on the detection result of the product and so on are the same asthose in the first embodiment. Note that gas containing CO₂ can be usedin place of the first electrolytic solution containing CO₂ in the secondembodiment.

The electrochemical reaction device 1B according to the secondembodiment illustrated in FIG. 3 includes an electrochemical reactioncell 10 including a reduction electrode 8, an oxidation electrode 9, adiaphragm 6, a first flow path 31 for allowing gas containing CO₂(simply described as CO₂ gas in some cases) or a first electrolyticsolution (cathode solution) containing CO₂ to flow therethrough, asecond flow path 32 for allowing a second electrolytic solution (anodesolution) containing water to flow therethrough, a first currentcollector plate 33 electrically connected to the reduction electrode 8,and a second current collector plate 34 electrically connected to theoxidation electrode 9. The first and second current collector plates 33,34 of the electrochemical reaction cell 10 are connected to the powersupply 11. Between the electrochemical reaction cell 10 and the powersupply 11, a voltmeter for monitoring the cell voltage is connected as areaction product detector 15. The reaction product detector 15 may be anammeter for monitoring the cell current, or the potential detectionmonitor or analyzer mentioned in the first embodiment.

The first flow path 31 is arranged to face the reduction electrode 8. Tothe first flow path 31, not-illustrated gas or solution tank, pump andso on are connected and configured such that the CO₂ gas or cathodesolution flows through the first flow path 31 and comes into contactwith the reduction electrode 8. The CO₂ in the CO₂ gas or cathodesolution passed through the reduction electrode 8 is reduced by thereduction electrode 8. A gas or solution containing the reductionreaction product of CO₂ is sent to the first product separator 12.Between the first flow path 31 and the first product separator 12, apressure regulator 14 is provided. The second flow path 32 is arrangedto face the oxidation electrode 9. To the second flow path 32,not-illustrated solution tank, pump and so on are connected andconfigured such that the anode solution flows through the second flowpath 32 and comes into contact with the oxidation electrode 9. The H₂Oin the anode solution passed through the oxidation electrode 9 isoxidized by the oxidation electrode 9. A solution containing theoxidation reaction product of H₂O is sent to the second productseparator 13.

In the electrochemical reaction device 1B in the second embodiment, thereaction product detector 15 monitors the cell current and therebydetects the composition and the amount of the reduction reactionproduct. In this case, the composition and the amount of the product canbe recognized indirectly from the cell voltage and current byinvestigating in advance the relation (dependence) between thecomposition and the amount of the product, and, the cell voltage andcurrent. The pressure regulator 14 is controlled based on the monitoringresult by the reaction product detector 15, whereby the pressure in thefirst flow path 31 is regulated. This adjusts the potential of thereduction electrode 8 to be a desired potential, and adjusts thecomposition and the production amount of the reduction reaction productto desired states. Accordingly, it becomes possible to suppress thesuccessive fluctuations in production amount and composition of thereduction reaction product of CO₂ due to the change in the appliedvoltage to the reduction electrode 8 to thereby enhance the availabilityand utility value of the reduction reaction product.

EXAMPLES

Next, examples and their evaluation results will be described.

Example 1

The electrochemical reaction device 1A having the configurationillustrated in FIG. 1 was manufactured. First, the electrochemicalreaction device 1A was used to control the pressure in the firstaccommodation part 3 to thereby carry out verification whether thepotential of the reduction electrode 8 was controllable. In thisverification experiment, the controller 16 was not operated, but thereading of the product by the reaction product detector 15 and theoperation of the pressure regulator 14 were manually operated.

As the electrochemical reaction cell, an acrylic reaction container wasused.

At the middle of the reaction container, an anion exchange membrane wasarranged to separate the reaction container into two chambers of a firstaccommodation part and a second accommodation part. As the reductionelectrode, the one obtained by Au plating on carbon paper was used. Asthe oxidation electrode, nickel mesh was used. For the first and secondelectrolytic solutions, a 0.5 M KHCO₃ aqueous solution was used. As thereference electrode, Ag/AgCl (3M NaCl) was used and inserted into thefirst accommodation part. Further, CO₂ was supplied to the firstaccommodation part, and CO₂ was supplementarily supplied to the secondaccommodation part. For the power supply, a DC stabilized power supplywas used for a verification experiment, and was connected to thereduction electrode and the oxidation electrode. For the reactionproduct detector for the product, a data logger capable of recordingvoltage was used. For the pressure regulator, a relief valve was used.

The potential dependence of the Faradaic efficiency of the productproduced in the reduction reaction of CO₂ was evaluated in advance by athree-electrode system measurement using a potentiostat. The gascomponents discharged from the reduction electrode side were analyzed bya gas chromatography apparatus. The gas components observed by the gaschromatography apparatus were CO, H₂, and CO₂. Then, the Faradaicefficiency over time of CO of the CO₂ reduced substance produced at thereduction electrode was calculated. The CO Faradaic efficiency wascalculated from the following expression. Note that the CO productionrate from the gas chromatography analysis result was used, and the valueobserved by the ammeter was used as the current value. Besides, thenumber of reaction electrons was set to 2.

$\begin{matrix}{{{CO}\mspace{14mu} {FARADIAC}\mspace{14mu} {{EFFICIENCY}\mspace{11mu}\lbrack\%\rbrack}} = \frac{\begin{matrix}\begin{matrix}{{CO}\mspace{14mu} {PRODUCTION}\mspace{14mu} {{RATE}\mspace{11mu}\left\lbrack {{mol}\text{/}s} \right\rbrack} \times} \\{{FARADAY}\mspace{14mu} {CONSTANT} \times}\end{matrix} \\{{NUMBER}\mspace{14mu} {OF}\mspace{14mu} {REACTION}} \\{ELECTRONS}\end{matrix}}{{CURRENT}\mspace{14mu} {{VALUE}\;\lbrack A\rbrack}}} & \left\lbrack {{Mathematical}\mspace{14mu} {Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The result is illustrated in FIG. 4. FIG. 4 shows that the Faradaicefficiency of CO being the reduced substance of CO₂ has potentialdependence and reaches the maximum value near an electrode potential of−1.2 to −1.3 V. Next, whether the potential of the reduction electrodewas controllable by controlling the pressure in the first accommodationpart was verified. As an operation method, the relief valve being thepressure regulator was arbitrarily throttled with a voltage of 3.0 Vapplied to the cell, and the change in the reduction electrode potentialat that time was observed. The results are illustrated in FIG. 5 andFIG. 6. FIG. 5 is a graph obtained by monitoring the pressure in thefirst accommodation part (cathode chamber) by the pressure sensor, andFIG. 6 illustrates the potential of the reduction electrode recorded onthe data logger being the product detection unit. It is found from FIG.5 and FIG. 6 that with an increase in pressure in the firstaccommodation part (cathode chamber), the electrode potential increasesin response to the change in pressure.

From the above verification result, it was finally verified that theelectrode potential of the reduction electrode was able to be changed bycontrolling the pressure in the first accommodation part in which thereduction electrode was arranged. The potential of the reductionelectrode is a factor of deciding the amount and the composition of theproduct, and therefore can suppress the fluctuations in productionamount and composition of the CO₂ reduced substance so as to stablyreduce CO₂ for a long period.

Based on the above result, a power supply derived from the renewableenergy was used as the power supply, and a reduction test of CO₂ wascarried out. In this event, the amount and the composition of theproduct at the reduction electrode were visually checked by thepotential monitor operated as the reaction product detector, and adetection signal thereof was sent to the controller so as to control thepressure regulator. As a result, though there was a fluctuation inpotential applied to the reduction electrode from the power supplyderived from the renewable energy, it was confirmed that the mount andthe composition of the product at the reduction electrode weresuppressed in fluctuation based on the regulation of the pressure in thefirst accommodation part.

Note that the above-described configurations in the embodiments areapplicable in combination, and parts thereof are also replaceable. Whilecertain embodiments have been described, these embodiments have beenpresented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An electrochemical reaction device comprising: areaction unit comprising a first accommodation part configured toaccommodate carbon dioxide, a second accommodation part configured toaccommodate an electrolytic solution containing water, and a diaphragmprovided between the first accommodation part and the secondaccommodation part; a reduction electrode configured to reduce thecarbon dioxide; an oxidation electrode configured to oxidize the water;a pressure regulator configured to regulate a pressure in the firstaccommodation part; a reaction product detector configured to detect atleast one of an amount and a kind of a substance produced at thereduction electrode; and a controller configured to control the pressureregulator based on a detection signal of the reaction product detector.2. The device according to claim 1, wherein the reaction productdetector comprises an analyzer configured to analyze the substanceproduced at the reduction electrode.
 3. The device according to claim 1,wherein the reaction product detector comprises an electrode potentialmonitor configured to monitor a potential of the reduction electrode. 4.The device according to claim 3, wherein the electrode potential monitoris connected to a reference electrode arranged in the firstaccommodation part, and is configured to monitor the potential of thereduction electrode through the reference electrode.
 5. The deviceaccording to claim 1, wherein the reaction product detector comprises avoltage and current monitor configured to monitor at least one of avoltage value and a current value of a current flowing between thereduction electrode and the oxidation electrode.
 6. The device accordingto claim 1, wherein the pressure in the first accommodation part isregulated to be 0.1 MPa or more and 6.4 MPa or less.
 7. The deviceaccording to claim 1, further comprising a second pressure regulatorconfigured to regulate a pressure in the second accommodation part. 8.The device according to claim 7, wherein a difference between thepressure in the first accommodation part and the pressure in the secondaccommodation part is regulated to be 0.5 MPa or less.
 9. The deviceaccording to claim 1, wherein the reaction unit comprises the firstaccommodation part configured to accommodate an electrolytic solutioncontaining the carbon dioxide in a manner to be in contact with thereduction electrode, and the second accommodation part configured toaccommodate the electrolytic solution containing water in a manner to bein contact with the oxidation electrode.
 10. The device according toclaim 1, wherein the reaction unit comprises a first flow pathconfigured to allow a gas or electrolytic solution containing the carbondioxide to flow therethrough in a manner to be in contact with thereduction electrode as the first accommodation part, and a second flowpath configured to allow the electrolytic solution containing water toflow therethrough in a manner to be in contact with the oxidationelectrode as the second accommodation part.